I have a Perl CGI program. myperlcgi.pl
Within this program I have the following:
my $retval = system('extprogram')
extprogram has a print statement within.
The output from extprogram is being included within myperlcgi.pl
I tried adding
> output.txt 2>&1
to system call but did not change anything.
How do I prevent output form extprogram being used in myperlcgi.pl.
Surprised that stdout from system call is being used in myperlcgi.pl
The system command just doesn’t give you complete control over capturing STDOUT and STDERR of the executed command.
Use backticks or open to execute the command instead. That will capture the STDOUT of the command’s execution. If the command also outputs to STDERR, then you can append 2>&1 to redirect STDERR to STDOUT for capture in backticks, like so:
my $output = `$command 2>&1`;
If you really need the native return status of the executed command, you can get that information using $? or ${^CHILD_ERROR_NATIVE}. See perldoc perlvar for details.
Another option is to use the IPC::Open3 Perl library, but I find that method to be overkill for most situations.
I have a perl script which takes 2 arguments as follows and calls appropriate function depending on the argument. I call this script from bash, but i want to call it from perl, is it possible?
/opt/sbin/script.pl --group="value1" --rule="value2";
Also the script exits with a return value that I would like to read.
The Perl equivalent of sh command
/opt/sbin/script.pl --group="value1" --rule="value2"
is
system('/opt/sbin/script.pl', '--group=value1', '--rule=value2');
You could also launch the command in a shell by using the following, though I'd avoid doing so:
system(q{/opt/sbin/script.pl --group="value1" --rule="value2"});
Just like you'd have to do in sh, you'll have to follow up with some error checking (whichever approach you took). You can do so by using use autodie qw( system );. Check the docs for how to do it "manually" if you want more flexibility.
If you want to capture the output:
$foo = `/opt/sbin/script.pl --group="value1" --rule="value2"`;
If you want to capture the exit status, but send script.pl's output to stdout:
$status = system "/opt/sbin/script.pl --group=value1 --rule=value2";
If you want to read its output like from a file:
open SCRIPT, "/opt/sbin/script.pl --group=value1 --rule=value2 |" or die $!;
while (<SCRIPT>) ...
Yes. You can use system, exec, or <backticks>.
The main difference between system and exec is that exec "executes a command and never returns.
Example of system:
system("perl", "foo.pl", "arg");
I use a Perl script to configure and spawn a compiled program, that needs a subshell configured a certain way, so I use $returncode = system("ulimit -s unlimited; sg ourgroup 'MyExecutable.exe'");
I want to capture and parse the STDOUT from that, but I need it forked, so that the output can be checked while the job is still running. This question comes close:
How can I send Perl output to a both STDOUT and a variable? The highest-rated answer describes a function called backtick() that creates a child process, captures STDOUT, and runs a command in it with exec().
But the calls I have require multiple lines to configure the shell. One solution would be to create a disposable shell script:
#disposable.sh
#!/bin/sh
ulimit -s unlimited
sg ourgroup 'MyExecutable.exe'
I could then get what I need either with backtick(disposable.sh) or open(PROCESS,'disposable.sh|').
But I'd really rather not make a scratch file for this. system() happily accepts multi-line command strings. How can I get exec() or open() to do the same?
If you want to use shell's power (that includes loops, variables, but also multiple command execution), you have to invoke the shell (open(..., 'xxx|') doesn't do that).
You can pass your shell script to the shell with the -c option of the shell (another possibility would be to pipe the commands to the shell, but that's more difficult IMHO).
That means calling the backtick function from the other answer like this:
backtick("sh", "-c", "ulimit -s unlimited; sg ourgroup 'MyExecutable.exe'");
The system tee with backticks will do this, no?
my $output = `(ulimit -s unlimited; sg ourgroup 'MyExecutable.exe') | tee /dev/tty`;
or modify Alnitak's backticks (so it does use a subshell)?
my $cmd = "ulimit -s unlimiited ; sg ourgroup 'MyExecutable.exe'";
my $pid = open(CMD, "($cmd) |");
my $output;
while (<CMD>) {
print STDOUT $_;
$output .= $_;
}
close CMD;
Expect should be used as you are interacting with your program: http://metacpan.org/pod/Expect
Assuming /bin/bash on your *nix matches something like bash-3.2$ the below program can be used to launch number of commands using $exp->send on bash console and output from each command can then be parsed for further actions.
#!/usr/bin/perl
use Expect;
my $command="/bin/bash";
my #parameters;
my $exp= new Expect;
$exp->raw_pty(1);
$exp->spawn($command);
$exp->expect(5, '-re', 'bash.*$');
$exp->send("who \n");
$exp->expect(10, '-re', 'bash.*$');
my #output = $exp->before();
print "Output of who command is #output \n";
$exp->send("ls -lt \n");
$exp->expect(10, '-re', 'bash.*$');
my #output = $exp->before();
print "Output of ls command is #output \n";
I am running unix commands in perl as
$cmd="ls -l";
$result=`$cmd`;
print $log_file $result;
but the output from execution is also printed on screen.
how to execute command without printing result on screen ?
The standard output stream isn't printed to screen -- it's captured to $result. But there is a second output stream called the standard error stream that programs can write to, and which also by default goes to the screen. Many programs use this stream for logging, or (in the case of ls) for writing error messages. To capture this in addition, use
$cmd = "ls -l 2>&1";
To discard it instead, use
$cmd = "ls -l 2>/dev/null";
I think there's a mistake. Try that in a shell:
perl -e 'my $x = `ls -l`;'
That doesn't produce any output.
it happens probably because $log_file is somehow attached to STDOUT, duplicated to it, something like:
open(my $log_file, ">&STDOUT")
Can someone please help me? In Perl, what is the difference between:
exec "command";
and
system("command");
and
print `command`;
Are there other ways to run shell commands too?
exec
executes a command and never returns.
It's like a return statement in a function.
If the command is not found exec returns false.
It never returns true, because if the command is found it never returns at all.
There is also no point in returning STDOUT, STDERR or exit status of the command.
You can find documentation about it in perlfunc,
because it is a function.
system
executes a command and your Perl script is continued after the command has finished.
The return value is the exit status of the command.
You can find documentation about it in perlfunc.
backticks
like system executes a command and your perl script is continued after the command has finished.
In contrary to system the return value is STDOUT of the command.
qx// is equivalent to backticks.
You can find documentation about it in perlop, because unlike system and execit is an operator.
Other ways
What is missing from the above is a way to execute a command asynchronously.
That means your perl script and your command run simultaneously.
This can be accomplished with open.
It allows you to read STDOUT/STDERR and write to STDIN of your command.
It is platform dependent though.
There are also several modules which can ease this tasks.
There is IPC::Open2 and IPC::Open3 and IPC::Run, as well as
Win32::Process::Create if you are on windows.
In general I use system, open, IPC::Open2, or IPC::Open3 depending on what I want to do. The qx// operator, while simple, is too constraining in its functionality to be very useful outside of quick hacks. I find open to much handier.
system: run a command and wait for it to return
Use system when you want to run a command, don't care about its output, and don't want the Perl script to do anything until the command finishes.
#doesn't spawn a shell, arguments are passed as they are
system("command", "arg1", "arg2", "arg3");
or
#spawns a shell, arguments are interpreted by the shell, use only if you
#want the shell to do globbing (e.g. *.txt) for you or you want to redirect
#output
system("command arg1 arg2 arg3");
qx// or ``: run a command and capture its STDOUT
Use qx// when you want to run a command, capture what it writes to STDOUT, and don't want the Perl script to do anything until the command finishes.
#arguments are always processed by the shell
#in list context it returns the output as a list of lines
my #lines = qx/command arg1 arg2 arg3/;
#in scalar context it returns the output as one string
my $output = qx/command arg1 arg2 arg3/;
exec: replace the current process with another process.
Use exec along with fork when you want to run a command, don't care about its output, and don't want to wait for it to return. system is really just
sub my_system {
die "could not fork\n" unless defined(my $pid = fork);
return waitpid $pid, 0 if $pid; #parent waits for child
exec #_; #replace child with new process
}
You may also want to read the waitpid and perlipc manuals.
open: run a process and create a pipe to its STDIN or STDERR
Use open when you want to write data to a process's STDIN or read data from a process's STDOUT (but not both at the same time).
#read from a gzip file as if it were a normal file
open my $read_fh, "-|", "gzip", "-d", $filename
or die "could not open $filename: $!";
#write to a gzip compressed file as if were a normal file
open my $write_fh, "|-", "gzip", $filename
or die "could not open $filename: $!";
IPC::Open2: run a process and create a pipe to both STDIN and STDOUT
Use IPC::Open2 when you need to read from and write to a process's STDIN and STDOUT.
use IPC::Open2;
open2 my $out, my $in, "/usr/bin/bc"
or die "could not run bc";
print $in "5+6\n";
my $answer = <$out>;
IPC::Open3: run a process and create a pipe to STDIN, STDOUT, and STDERR
use IPC::Open3 when you need to capture all three standard file handles of the process. I would write an example, but it works mostly the same way IPC::Open2 does, but with a slightly different order to the arguments and a third file handle.
Let me quote the manuals first:
perldoc exec():
The exec function executes a system command and never returns-- use system instead of exec if you want it to return
perldoc system():
Does exactly the same thing as exec LIST , except that a fork is done first, and the parent process waits for the child process to complete.
In contrast to exec and system, backticks don't give you the return value but the collected STDOUT.
perldoc `String`:
A string which is (possibly) interpolated and then executed as a system command with /bin/sh or its equivalent. Shell wildcards, pipes, and redirections will be honored. The collected standard output of the command is returned; standard error is unaffected.
Alternatives:
In more complex scenarios, where you want to fetch STDOUT, STDERR or the return code, you can use well known standard modules like IPC::Open2 and IPC::Open3.
Example:
use IPC::Open2;
my $pid = open2(\*CHLD_OUT, \*CHLD_IN, 'some', 'cmd', 'and', 'args');
waitpid( $pid, 0 );
my $child_exit_status = $? >> 8;
Finally, IPC::Run from the CPAN is also worth looking at…
What's the difference between Perl's backticks (`), system, and exec?
exec -> exec "command"; ,
system -> system("command"); and
backticks -> print `command`;
exec
exec executes a command and never resumes the Perl script. It's to a script like a return statement is to a function.
If the command is not found, exec returns false. It never returns true, because if the command is found, it never returns at all. There is also no point in returning STDOUT, STDERR or exit status of the command. You can find documentation about it in perlfunc, because it is a function.
E.g.:
#!/usr/bin/perl
print "Need to start exec command";
my $data2 = exec('ls');
print "Now END exec command";
print "Hello $data2\n\n";
In above code, there are three print statements, but due to exec leaving the script, only the first print statement is executed. Also, the exec command output is not being assigned to any variable.
Here, only you're only getting the output of the first print statement and of executing the ls command on standard out.
system
system executes a command and your Perl script is resumed after the command has finished. The return value is the exit status of the command. You can find documentation about it in perlfunc.
E.g.:
#!/usr/bin/perl
print "Need to start system command";
my $data2 = system('ls');
print "Now END system command";
print "Hello $data2\n\n";
In above code, there are three print statements. As the script is resumed after the system command, all three print statements are executed.
Also, the result of running system is assigned to data2, but the assigned value is 0 (the exit code from ls).
Here, you're getting the output of the first print statement, then that of the ls command, followed by the outputs of the final two print statements on standard out.
backticks (`)
Like system, enclosing a command in backticks executes that command and your Perl script is resumed after the command has finished. In contrast to system, the return value is STDOUT of the command. qx// is equivalent to backticks. You can find documentation about it in perlop, because unlike system and exec, it is an operator.
E.g.:
#!/usr/bin/perl
print "Need to start backticks command";
my $data2 = `ls`;
print "Now END system command";
print "Hello $data2\n\n";
In above code, there are three print statements and all three are being executed. The output of ls is not going to standard out directly, but assigned to the variable data2 and then printed by the final print statement.
The difference between 'exec' and 'system' is that exec replaces your current program with 'command' and NEVER returns to your program. system, on the other hand, forks and runs 'command' and returns you the exit status of 'command' when it is done running. The back tick runs 'command' and then returns a string representing its standard out (whatever it would have printed to the screen)
You can also use popen to run shell commands and I think that there is a shell module - 'use shell' that gives you transparent access to typical shell commands.
Hope that clarifies it for you.